Applications of magnetic bearings to rotary machines have become much more widespread in recent years because of the major advantage procured by them being able to operate directly in the process gas of the machine in question, without any sealing. Thus, in non-limiting manner, magnetic bearings are to be found in turboexpanders, in refrigerator compressors, in electric motors for driving compressors, etc.
In applications in common use, all of the magnetic circuits are based on silicon-iron. The magnetic laminations forming the laminated magnetic material of such circuits thus have the advantage of presenting magnetic characteristics that are well defined and that are guaranteed by their suppliers, in particular a limited hysteresis cycle characterized by its coercive field strength and its remanent magnetic flux density, and high magnetic permeability and high saturation.
However, for more particular applications, in particular for processing natural gas and for applications in acid, particle-carrying, or corrosive environments such as wet hydrogen sulfide (H2S) or wet carbon dioxide (CO2), it is impossible to use such magnetic laminations because they are incompatible with such an environment. The same applies to the stator coils of the bearings and of the detectors in such active magnetic bearings because they are not sealed off from the surrounding environment.
That is why, in patent EP 1 830 081, the Applicant has proposed active magnetic bearings in which the stator of the bearing is protected by a jacket of precipitation hardening martensitic stainless steel, and the stator of the detector is protected by a jacket of austenitic stainless steel, and in which the laminated magnetic material of the rotor is also made of precipitation hardening martensitic stainless steel.
FIG. 1 shows an example of a radial active magnetic bearing of a compressor as described in the above-mentioned patent. There can be seen a rotor 2 of a rotary machine that is designed to be in contact with a process gas, which gas may be acid, corrosive, or a carrier of particles.
A bearing armature 3 of laminated magnetic material is applied to the rotor 2. This armature 3 is made of 17-4 PH precipitation hardening martensitic stainless steel.
A detector armature 4, likewise in 17-4 PH precipitation hardening martensitic stainless steel, is fitted on the rotor 2, in the vicinity of the bearing armature 3.
An airgap 5 presenting thickness that lies for example in the range 0.3 millimeters (mm) to 0.5 mm is provided between firstly the peripheral portion of the rotor 2 fitted with the bearing armature 3 and the detector armature 4, and secondly a first jacket 6 constituting a bearing jacket and a second jacket 7 constituting a detector jacket.
The first jacket 6 is welded in leaktight manner to parts 8, 9 co-operating with said first jacket to constitute a leaktight housing 10 containing the elements constituting the stator of the magnetic bearing 11, i.e. electromagnet windings 12 associated with a yoke 13 of silicon-iron laminated magnetic material. The first jacket 6 is made of 17-4 PH precipitation hardening martensitic stainless steel.
A potting compound 14 is introduced into the inside of the leaktight housing in order to fill its empty spaces around the bearing electromagnet windings 12 totally, and improve its mechanical strength.
A position detector 15 of the electromagnetic type comprises a stator that is disposed in a second housing 16 distinct from the first housing 10 and closed by the second jacket 7 that is welded in leaktight manner to the parts 17, 18 of the second housing. The stator of the position detector comprises electromagnet windings 19 associated with a yoke 20 of silicon-iron laminated magnetic material.
The second jacket 7 is made of austenitic stainless steel of the American Iron and Steel Institute (AISI) 304, 304L, 316, or 316L type.
In the same way as for the bearing stator, a potting compound 21 is introduced into the inside of the detector housing 16 in order to fill its empty spaces and improve its mechanical strength.
Preferably, the first leaktight housing 10 containing the bearing stator and the second leaktight housing 16 containing the detector stator are interconnected in leaktight manner in a zone that is remote from the first and second jackets 6, 7.
The electromagnet windings 12, 19 of the bearing and of the detector are connected to electronic control circuits 22 that, as shown, can be placed outside the housings of the bearing.
The jackets put into place in this way isolate the magnetic circuits of the stator from the gaseous environment, thereby making it possible to use conventional magnetic laminations based on silicon-iron for said circuits. At the rotor, the use of laminations made of precipitation hardening martensitic stainless steel procures this resistance to the gaseous environment directly without any other protection.
However, the magnetic properties of precipitation hardening martensitic stainless steels are far from ideal, reducing the performance of the magnetic bearing. Its low magnetic flux density at saturation puts a limit on the static and dynamic load capacity of the bearings due to limited magnetic flux density, and requires the lengths of the bearings to be increased for the same load capacity, and its wide hysteresis cycle causes large iron losses, which are about ten times greater than the losses caused in a conventional rotor equipped with silicon-iron magnetic laminations, resulting in the magnetic bearing being heated to a considerable extent, and requiring said bearing to be cooled to a considerable extent.
In addition, in “oil and gas” environments, with applications required to comply with the ANSI/NACE MR0175/ISO15156 Standard “Petroleum and Natural Gas Industries—Materials for Use in H2S-containing Environments in Oil and Gas Production”, the use of precipitation hardening martensitic stainless steels requires the welds to be heat-treated, typically by high-temperature annealing performed at in the range 500° C. to 800° C. (of the H1150D or H1150M type), such heat-treatment being incompatible with the material of the windings of the bearing stator 12 that, conventionally, cannot withstand temperatures higher than 250° C.
In order to achieve such compatibility, U.S. Pat. No. 7,847,454 thus proposes using a two-material bearing jacket made up of a magnetic central portion made of precipitation hardening martensitic stainless steel, on either side of which non-magnetic inserts are fastened. Thus, by also equipping the housing with non-magnetic inserts, it is possible, once those inserts have been welded both to the jacket and to the housing, and once all of those welds have been subjected to high-temperature annealing, to place the windings in the housing and then to weld the non-magnetic inserts together, without applying any particular heat treatment, and at a temperature compatible with the material of the windings, in order to make the housing leaktight.
Unfortunately, those welding operations are lengthy and complex, and are thus costly. They are also major sources of leakage or of manufacturing defects, in particular at the two-material bearing sleeve which, since it is of very small thickness, makes it particularly difficult to weld to the inserts.